The present invention is directed to a spark plug insulator having high dielectric strength, high density, fine grain, and an optical property that allows the passage of light.
An exemplary spark plug is illustrated in FIG. 1 having an outer shell 20 secured to an insulator 30. A center electrode 50 and terminal 40 are at least partially secured within the insulator 30. The insulator 30 insulates the charged terminal 40 and center electrode 50 from the ground electrode 22. The insulator 30 also separates the terminal 40 and center electrode 50 from outside electrical interferences.
As manufacturers continually increase the complexity and reduce the size of internal combustion engines, there is a strong need for spark plugs that are smaller and occupy less space. Currently, the size of the spark plug, particularly the diameter of the spark plug, is limited from further reduction due to the required dielectric strength of the insulator over the service lifetime of the plug. The dielectric strength is directly related to required thickness for the walls of the insulator. Another factor limiting size reduction is that more manufacturers are demanding a longer service lifetime from spark plugs such as requesting 100,000 mile, 150,000 mile, and 175,000 mile service lifetimes from spark plugs. The longer the desired service lifetime, the higher the required dielectric strength. In the past, the common way to increase the service lifetime or dielectric strength of a spark plug was to increase the thickness of the walls of the insulator. However, the current demand for more compact spark plugs for modern engines prevents or limits the use of thicker walled insulators.
The recent movement to electromechanical valve actuators in place of cam shafts to improve fuel efficiency and power is also expected to further increase the demand for thinner and smaller spark plugs. The demand for higher dielectric strength is also being driven by recent developments and trends in modern internal combustion engines. To improve fuel economy and increase performance, engines are being designed with higher compression and turbocharged engines are becoming more common. To make a spark jump the spark gap under higher compression requires higher voltage and therefore higher dielectric strength in an insulator. Therefore, an insulator for a spark plug having increased dielectric strength while having reduced wall thickness and size is needed.
Modern spark plugs are typically formed from an alumina composition with other proprietary additives unique to a particular spark plug manufacturer. When formed, the alumina with the proprietary components typically forms a ceramic insulator that is white and not transparent. In fact the inventors are not aware of any current spark plugs where the insulator allows transmission of light so that the insulator visibly shows the spark within the combustion chamber under normal illumination, much less in sunlight. While some past insulators formed from an amorphous material, such as glass, were transparent, these insulators do not meet the dielectric strength required by modern internal combustion engines, and especially the current size requirements as a much greater wall thickness of these prior transparent insulators was required even under reduced dielectric strength requirements. In diagnosing various engine problems, it would be desirable to see in the cylinder or at a minimum see the combustion to determine if there is a problem with the spark occurring in a cylinder. Currently, to determine if there is a problem with the electrical ignition, spark plug wires, or spark plugs on a vehicle, special tests are required. Therefore, it is desirable to see the spark occur in the combustion chamber of the cylinder while yet retaining a spark plug having high dielectric strength and desirable mechanical qualities, including the diameter of the insulator.
The insulator 30 is traditionally fired and glazed to provide a smooth surface on the terminal portion 12 of the insulator 30. Glazing is required to prevent flash-over which may occur on unglazed insulators 30 on the terminal portion 12. To reduce manufacturing costs and time, it is desirable to not glaze the insulators, however, currently, without additional processing steps, the outer surface of the insulator cannot be made with the required smoothness to prevent flash-over and therefore must be glazed.
The firing end portion or core nose 14 of the insulator 30 is typically not glazed due to the expense of the glazing process, as flash-over is generally not a problem for this portion of the spark plug. Therefore, the firing end portion or core nose of the insulator 30 traditionally has a rough surface that may attract deposits from the combustion process in the cylinder, which may detrimentally affect firing of the spark plug. In engines that are not correctly tuned, are not operating efficiently, or have mechanical problems such as oil leakage into the cylinder during the combustion process, these deposits can quickly and significantly build up to eventually create an electrical conduit between the center electrode 50 and the shell 20 of lower resistance then jumping the spark plug gap, which in turn prevent the spark plug from providing the proper sparking profile to efficiently ignite the gases in the cylinder. Two-cycle engines may also build up deposits due to the oil present in the cylinder during combustion. Therefore, it is desirable to develop a material for use as a spark plug insulator that has a smoother surface that does not require glazing or other finishing processes.